The Mutator is an entity that takes one or more Inputs and generates a new instance of Input derived by its inputs.
Mutators can be composed, and they are generally linked to a specific Input type.
There can be, for instance, a Mutator that applies more than a single type of mutation to the input. Consider a generic Mutator for a byte stream, bit flip is just one of the possible mutations but not the only one, there is also, for instance, the random replacement of a byte of the copy of a chunk.
There are also mutators that always produce valid inputs, say a mutator that generates valid JSON or code, but these grammar based mutators need a grammar to work.
In LibAFL, Mutator is a trait.